1. Field of the Invention
The invention relates to a valve seat design to reduce cavitation of liquid and erosion in a flow control valve generally used as a choke.
2. Description of the Prior Art
The removable internal parts of a valve are generally referred to as the "trim" and generally function to proportion the valve orifice area to control flow through the valve. Erosion of the valve trim is a major problem in valves controlling flow of high velocity liquid. Cavitation may occur if the pressure of liquid flowing through restricted passages becomes less than the vapor pressure of the liquid at the operating temperature. Flow control valves usually experience cavitation when a high pressure drop is created across a single point of pressure reduction.
Flow of fluid through an orifice is narrowed downstream of the orifice to form a vena contracta, which is the location of highest velocity of fluid through the valve and the location of the lowest pressure. As fluid velocity decelerates downstream of the orifice, pressure is regained and the vapor bubbles collapse or implode violently, expending energy which is absorbed by the valve causing cavitation related wear, fatigue and eventual failure.
Various trims which reduce cavitation are shown in ISA HANDBOOK OF CONTROL VALVES, 2nd Ed, 1976 Instrument Society of America. In cage hole designs to reduce cavitation, a cylindrical cage member with circumferentially opposed opening directs flow to impinge upon itself. These designs do reduce cavitation; however, the cylindrical trim elements with openings therein tend to foul and create localized erosion and flow cutting. In cascading designs trim is designed to create a series of smaller pressure drops to reduce cavitation. However, these cascading type trims tend to reduce flow capacity and are expensive to manufacture.
In addition, heretofore it has been necessary to select valves having trim of various designs depending upon the velocity of fluid and field conditions under which a flow control valve is to be used for oil field service.